There are various levels in the LNH Imprint's cosmology.
The Looniverse is a cosmos superficially similar to our own. It contains the Classic LNH's home planet, the Loonivearth; other planets in its solar system, like Nox; and the denizens of worlds and galaxies beyond that, like the Dorfian Empire and the Dvorakian Empire. See also Looniversal Physics.
The Looniverse is just one Universe in a vast infinity of alternate Looniverses, such as the world of the Oddball Legion, the various Evil LNH Looniverses, Looniverse-Y, Looniverse-20, and many others.
The Omnilooniverse itself is located on the newsgroup rec.arts.comics.creative, which also has a number of other Imprint Multiverses, like the Superhuman Worlds, the Barrier-locked ASH universes, and the Superguy Altiverses. Sometimes stories feature travel to (or from) universes located in other newsgroups; these universes are sometimes said to belong together in the Usenetverse, which in turn is part of the Internetverse.
The Astral Plane
The Astral Plane links many of these universes (possibly all of them), and travel between them can be accomplished by the people who know the World's Address. It may or may not stretch beyond the Usenetverse, depending on who you ask.
The Real World
The Looniverse is a client world to the Real World. It draws strength and existence from it. Through dimensional sympathy and the power of storytelling, events in each affect the other.
Despite the many dimensional travelers in the LNH, along with characters like Fourth Wall Lass, who can traverse the Fourth Wall, no denizen of the Looniverse has ever visited the Real World; however, the Mechanical Author came closest before he was stopped. Theoretically, Fourth Wall Lass's powers after they were boosted with the Rung of Revamp could; however, she believes this could potentially destroy the universe. The Existents, however, say that any attempt to enter the Real World would simply put you in a version of Real World Minus One.
Speaking of which, Real World Minus One is a version of the Real World that can be accessed by fictional characters, but even this is difficult.
The Fifth Wall and Beyond
The Fifth Wall is the barrier that separates the Real World from the unknown and perhaps unknowable reality that lies beyond even that. Somehow, the Mister Paprika Company's Omaha Project managed to create a crack in the Fifth Wall; Doctor Net.ropolis threw herself into it and returned with cosmic knowledge, leading her to build the Mechanical Author.